1. Field of Use
These teachings relate generally communications systems such as wireless data or telephone systems. More particularly, the invention relates to compensating phase error and phase ambiguity in digital data demodulation systems.
2. Description of Prior Art
A variety of multiple access communication systems have been developed for transferring information among a large number of system users. Techniques employed by such multiple access communication systems include time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes, such as amplitude companded single sideband (ACSSB), the basics of which are well known in the art.
Transmission over terrestrial, global, and extraterrestrial distances requires the use of modulated carriers and, for modern applications, Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM) are often used to exploit the advantages offered by digital modulation techniques.
Yet, as is known in the art, the coherent reception of a quadrature-modulated signal requires the demodulation circuit to unambiguously determine the transmitted signal phase. The phase as observed by the demodulator is determined from orthogonal components of the received signal referred to as the in-phase (I) channel and the quadrature (Q) channel. QPSK modulation uses the two transmitted bits to select one of four possible phases: ±45° or ±135°. The phase recovery is typically done in two steps: a sub-quadrature phase error resolution step and then a quadrature error phase resolution step.
Sub-quadrature phase error may result from mismatches between the transmitter and the receiver components, e.g., clocks or oscillators, relative platform motion, and environmental conditions along the signal path. The sub-quadrature phase error is determined by generating a phase error with respect to the closest of the four allowed transmission phases and then using a tracking loop to minimize the error provides sub-quadrature phase resolution. This is equivalent to removing the data by mapping the received signal to the first quadrant of the I/Q plane. Prior art approaches generally perform or approximate an Arc Tan operation using the I and Q data to determine the sub-quadrature phase error. The technique is well known in the art and need not be discussed here. However, it will be appreciated by those skilled in the art that the technique of performing an Arc Tan operation is computationally intensive and requires significant hardware recourse.
The method of sub-quadrature phase resolution described above may lock on to the correct phase or one of the other three incorrect phases. Prior art approaches correlate the output of the sub-quadrature phase tracking loop to a known data pattern to resolve the remaining quadrature phase ambiguity. This prior art technique and similar techniques require complex system hardware and/or software due to the extra correlation and rotation circuitry required.
Therefore it is desirable to compensate for sub-quadrature phase error and quadrature phase ambiguity with one phase detector, which makes more efficient use of hardware resources.